The invention generally relates to a sensor or hot wire anemometer for measuring the flow of a gas, especially for infrared gas analysis apparatus. More particularly, the invention relates to a microrheoscopic detector composed of an elongated, current-carrying heating conductor oriented perpendicularly relative to the direction of flow of the gas and elastically suspended at at least one side and composed of resistance thermometers on opposite sides of the heating conductor in the influencing region of the heating conductor, the resistance thermometers being composed of thin wires and lying in the same plane as the heating conductor, the resistance thermometers being located essentially parallel to and at a distance from the heating conductor, and having ends connected to intermediate wires.
In an article entitled 37 Reverse Flow Sensing Hot Wire Anemometer," that is published in Journal of Physics E; Scientific Instruments 1972, Vol. 5, No. 9, pp. 849-851 (Sept. 1972), the teachings of which are fully incorporated herein by reference, there is disclosed a microrheoscopic detector including the resistance thermometers that are held at both sides of a heating conductor by leads, but independently of this heating conductor. The respective planes in which the resistance thermometers and the heating conductor are situated are oriented perpendicularly relative to one another, this alone making it possible to secure the lead wires to the heating conductor, while requiring acceptance of further disadvantages. Due to the perpendicular arrangement of the resistance thermometers relative to the heating conductor, essentially only one flow direction is acquirable, namely, the flow perpendicular to the resistance thermometers and the heating conductor. Since the wires are separated by a great distance between one another over the major part of their length, the sensor has a high time constant that is unacceptable for numerous applications. Moreover, the measured signal is overridden by high noise signals, particularly given low flow rates. As proceeds from the article, the known system is suitable for very high flow rates of about 25 m per second, i.e. roughly motor vehicle speed.
In German application DE-AS 20 52 645, there is disclosed a thermal-electric anemometer including two thermal elements, one being in communication with a heating wire. However, such a measuring instrument is only suitable for relatively high flow rates in the range between 0.5 and 10 m per second. The purpose of the anemometer is to measure air speeds of the type that appear behind a motor vehicle radiator, or at the discharge opening of a heater, an air cooler, or a defroster.
Some physical measuring methods are concerned with measuring extremely low, pulsating pressures or volume streams in gases that are several orders of magnitude smaller than the above-described anemometers. Included among these, for example, are infrared gas analysis, leakage measurements in low sensitivity regions, respiratory measurements in medicine, or other measurements having extremely low gas consumption on the basis of a physical event.
Microrheoscopic detectors of the type described in the first paragraph are referred to as hot wire anemometers and can be used for the measurement of gas volume flows. A high speed microrheoscopic detector for gases is defined as a means of a flow measuring system for extremely small volume streams the functioning elements of which are constituted by two or more solid structures that are temperature-coupled via a gas path; a heated solid state structure that, for example, can be a relatively inert, massive part, generates a hot cloud in the gas to be measured, this hot cloud being capable of being described by an isotherm field. A forced flow deforms the gas cloud or, respectively, the isotherm field. A flow-proportional signal can be generated within limits by one or more low-mass temperature sensors that are arranged inside the gas cloud.
The demand for low-mass inertia temperature sensors leads, for example, to resistance thermometers composed of extremely thin resistance wires whose diameter lies between about 0.5 and 5 .times.10.sup.-3 mm. A low-mass inertia is required so that the resistance thermometers have an adequate resolution given pulsating gas flows on the order of magnitude between about 10 and 50 Hz.
When measurements are undertaken in a flow sensor for very minimal flows (i.e., microflows), care must also be taken to ensure that no free convection whatsoever can occur within the sensor that is contrary to the forced convection of the measuring effect. This requires dimensions of the active conductor volume of about 1 mm.sup.3 and less. Insofar as is possible, the length of the resistance thermometers should be between 0.4 and 1.5 mm.
A microrheoscopic detector produced in accordance with the afore-mentioned demands and design rules, is a product of extreme precision manufacturing involving stringent requirements of the precision of manufacture. High production costs with a considerable corresponding high wage expenditure are thus necessarily incurred in connection therewith.
In a dissertation entitled "Schnelle Messfuehler Fuer Kleine Gasstroeme" , and presented on Dec. 9, 1974 by Dr. Guenter Schunck of the University of Karlsruhe, there was disclosed a microrheoscopic detector of the type described in the first paragraph. In such microrheoscopic detector, a heating conductor and two resistance thermometers ar suspended independently of one another by lead wires, these lead wires allowing a longitudinal thermal expansion as a result of their resilient construction. While such a microrheoscopic detector completely satisfies the technical requirements and metrological demands made of it, experience has shown that it is sensitive to vibrations and is expensive to manufacture. Destruction of the sensitive resistance thermometers occasionally occurs due to the vibrations that are unavoidable in some measuring instruments. Problems also arise during assembly since the resistance thermometers must be individually secured to fused lead wires, and care must be taken to ensure that they are not mechanically over-stressed.
Finally, U.S. Pat. No. 4,154,087, the teachings of which are fully incorporated herein by reference, discloses a microrheoscopic detector for gases that includes an elongated, current-carrying conductor that is positioned perpendicularly relative to the direction of flow of a gas and is suspended elastically at at least one side and is further composed of respective resistance thermometers arranged in the influencing region of the heating conductor both preceding and following said heating conductor in the flow direction, these resistance thermometers being composed of thin wires, being held at their ends between thicker lead wires, lying in the same plane as the heating conductor, and positioned essentially parallel to and at a distance from the heating conductor, and having each of their two ends merging into the ends of lead wires, whereby the lead wires can be carried by the heating conductor. Experience has shown that this known microrheoscopic detector is particularly insensitive to vibrations and enables manufacturing thereof with relatively low waste due to rejects.